= Solution
\b[A useful idealization is a thin <sedimentary cover> over a <felsic> upper <continental crust>, an intermediate middle <continental crust>, and a more <mafic> lower <continental crust>, ending at the <Mohorovičić discontinuity> at about 35–40 km.] The boundaries within the <continental crust> are approximate compositional divisions, not three universal sharp interfaces.
\Image[/past-exam-of-the-plate-tectonics-course-of-the-university-of-oxford/2024-continental-crust.png]
{title=An idealized modern continental crustal column}
{description=An original schematic for a 38 km thick continental crust. Depth ranges illustrate a compositional tendency; individual provinces have different lithologies and boundaries.}
* At the surface, a variable, commonly kilometre-scale <sedimentary cover> contains <sediments> and <sedimentary rocks>. Exposed crystalline basement can instead occur at the surface.
* In the upper roughly 15 km, representative <rocks> are <granite>, <granodiorite> and compositionally related <gneiss>, with <quartz> and <feldspar>. <Sedimentary rocks>, <metamorphic rocks> and volcanic <rocks> make this layer heterogeneous.
* At roughly 15–25 km, intermediate <gneiss> and <amphibolite>-facies assemblages become important. <Diorite> to <granodiorite> compositions and <amphibole>- and <plagioclase>-bearing <rocks> are representative.
* At roughly 25–38 km, <mafic granulite> and <gabbro>-derived <metamorphic rocks> are a useful average, with <plagioclase>, <pyroxene> and sometimes <garnet>. Some provinces instead have substantially intermediate or <felsic> lower <continental crust>.
* Below the <Mohorovičić discontinuity> is <peridotite> of the <mantle>, dominated by <olivine> and <pyroxene>. This is not a fourth crustal layer.
The important trend is increasing average <mafic> character with depth, rather than a change from <granite> directly to <peridotite>. <Granulite> describes a <metamorphic facies>, not a unique chemical composition. The synthesis in <Nature and composition of the continental crust: A lower crustal perspective by Roberta L. Rudnick and David M. Fountain> combines seismic and petrological evidence for these distinctions.
Back to article page
